The invention relates to a method for controlling the operation of an internal combustion engine that is operated, in at least one operational range, with a homogeneous charge compression ignition mode, combustion history being monitored and evaluated by measuring at least one parameter relevant to combustion selected from the group comprising the start of combustion, the duration of combustion and the rate of combustion, and at least one parameter relevant to combustion being controlled, for subsequent combustion events, on account of the evaluation of the combustion.
The combustion of an auto-ignited lean fuel-air mixture has the advantage that extremely low NOx and soot emissions are obtained on account of the homogeneous distribution of concentration and temperature. This process is known as HCCI combustion (Homogeneous Charge Compression Ignition). HCCI combustion results in low NOx emissions, which is due to the fact that combustion is initiated at multiple ignition sites, the temperature of the combustion process being relatively low as a result thereof. For HCCI combustion, gasoline presents great advantages over diesel fuel on account of its low autoignition quality and the lower boiling range of between approximately 30xc2x0 and 190xc2x0. The compression ratio may be raised to values similar to those in a diesel engine of about 15 to 17.
On a compression-ignited Otto-cycle engine, the timing of ignition of the mixture can no longer be initiated by the actual ignition event occurring at the spark plug as this is the case with a spark-ignited Otto-cycle engine; in this mode of operation, only the in-cylinder charge, and more specifically the composition of the charge in the combustion chamber like pressure, temperature and fuel/air ratio xcex, are important for the commencement of combustion.
Internal combustion engines operating on compression ignition principles rely on increasing the temperature and the pressure in the combustion chamber during the compression phase to ignite the fuel/air mixture. In order for the ignition to indeed take place though, the in-cylinder charge must be conditioned in the right way at the beginning of the compression phase. The compression ratio xcex5 of the internal combustion engine and the mixing temperature Tmix of the in-cylinder charge at the time at which all of the gas exchange valves are closed and the compression phase starts thereby substantially influence the start of combustion. A high compression ratio has the advantage that the charge temperatures need not be so high at the beginning of the compression stroke as higher end temperatures can be achieved because of the smaller end volume of compression.
One possibility to influence combustion consists in changing the temperature of the intake air, which substantially influences the temperature of the in-cylinder charge. The fact is that the combustion increasingly approaches the top dead center of ignition as the temperature of the intake air and, as a result thereof, the mixing temperature of the in-cylinder charge at the beginning of the compression phase increases. In order to permit HCCI operation under normal boundary conditions, i.e., at intake air temperatures ranging from about 25xc2x0 C. to 30xc2x0 C., very high exhaust gas recirculation rates on the order of more than 50% are required to provide the appropriate initiation conditions for the in-cylinder charge.
The publications WO 98/07973 A1 and WO 98/10179 A2 disclose methods for controlling combustion on internal combustion engines operating under HCCI conditions.
It is the object of the invention to propose a simple method of controlling the combustion process in an internal combustion engine relying on homogeneous autoignition of a fuel-air mixture for operation.
According to the invention this is achieved by varying the level of turbulence in the combustion chamber to control combustion, the level of turbulence being temporarily raised to stabilize combustion.
In a particularly simple variant of the invention there is provided that, to raise the level of turbulence, the opening timing of at least one intake valve is advanced toward the top dead center position of ignition. The invention thereby makes use of the fact that, when the intake valve opens early, the combustion takes place at a much later stage than it does when the intake valve opens later, with all other parameters kept constant. To stabilize combustion by way of the level of turbulence is particularly suited for a fast change of the operating point in dynamic operation, i.e., for transition from the homogeneous charge spark ignition mode to the homogeneous charge compression ignition mode. A combustion control device making sure that combustion takes place in each engine cycle is provided for this purpose. A parameter relevant to combustion indicative of the situation of the last combustion, preferably the 50% mass fraction burned, is supplied to a combustion control device, and the time when the intake valve is to be opened is primarily influenced to stabilize combustion. Alternatively, it is also possible to achieve the level of turbulence in varying the valve overlap or in varying the valve timing of the exhaust valve.
Furthermore, one of several intake manifolds in each cylinder may be cut off to increase the level of turbulence. It is also possible to activate a swirl adjusting device in the intake manifold for the same purpose of increasing the level of turbulence.
In addition to temporarily increase the level of turbulence, there may also be provided to temporarily activate spark ignition, preferably in a region of a crank angle of approximately 50 degrees to 10 degrees before top dead center, in order to influence the combustion process as spark ignition causes preliminary reactions to take place in the combustion chamber, thus varying the combustion situation.
In order to allow combustion control by means of the combustion stabilizer, characteristic diagrams need to be filed in an engine control device for stationary motor operation, the required mass of exhaust gases to be recirculated and the mass of fresh intake air needed being read out from these diagrams for the corresponding stationary operating point. In the simplest case the diagrams can contain the gas exchange valve parameters such as opening timing and duration of opening for the respective one of the operating points. These characteristic diagrams serve as pilot control and are related to standard ambient conditions. If the temperature differs from the standard ambient temperature, the pilot control is rectified for the angle of opening of the intake valve by means of a characteristic curve of temperature. All other disturbance variables, such as a fast change of the load point for example, are rectified by the combustion control device.
The combustion control device at first detects the actual operating point of the internal combustion engine by way of the actual engine speed and the indicated mean effective pressure measured prior to determining the set value for the combustion situation in the next step. Said set value can be a constant value for the entire operational range or adopt different values in function of the operating point. Next, the angle of the electrical opening of the intake valve obtained at the desired operating point under stationary conditions is read out of a diagram. Thereupon, the value of this angle is rectified by way of a temperature-dependant characteristic curve and is then available as a pilot value for the combustion control device. The control deviation is obtained by means of the reference combustion situation for the stationary case and of the value of the 50% mass fraction burned, which is the mean of z working cycles (wherein z may adopt values comprised between 1 and 10), said control deviation being then delivered to a control device. The level of turbulence in the combustion chamber is increased on account of the control deviation. Once the combustion stabilization is complete, the level of turbulence is restored, in a further step, to the value corresponding to the respective one of the stationary operating points of the engine.